MPPT versus Switched Regulation (PWM) – Off Grid Solar

Date: October 21, 2012

The debate about both Maximum Power Point Tracking (MPPT) versus Switched Regulation (or referred as Pulse Width Modulation – PWM) is worth a discussion if you are either not sure what both these types of solar charging methods are, or if you are intending on making a decision on purchasing a Stand Alone (or Remote Area, Off Grid power system).

Many manufacturers’ claim MPPT’s gain up to 30% more power than from conventional switched solar regulators (PWM) we’ll investigate whether this is true and whether the cost of purchasing a MPPT over a PWM is actually worth the investment – as MPPT usually cost more than a Switched Regulator of same Current Rating.

Switched solar regulators have been around for over 30 years and have proved to be a reliable means of preventing batteries from over-charging in Off Grid solar. I personally was reluctant to use a MPPT for some time as the advantages seemed quite insignificant compared to the actual cost. Futhermore, the reliability and longevity of switched regulators seemed to be an overall advantage, whereas MPPTs are quite new in terms of actual use.

Why would a device take it’s place be necessary when infact switched regulators are perfectly suited to their job?

Well over a decade of field experience with MPPTs has lead to an increased reliability of these devices. Furthermore, they do have increased yields in electricity harvesting over switched regulators, however this is dependent on the type of system you have and possibly even the geographic location of the Off Grid solar system.

Switched solar regulators work by connecting the voltage and current produced by the solar panels directly the battery bank, an electrical load is connected to the Off Grid solar system it is either ‘taken’ from the battery bank or solar panels, or a combination of these. A switched solar regulator will allow both voltage and current (voltage x current = power) to enter or ‘recharge’ the battery bank.

If there is a higher voltage from the solar array than the battery bank, the switched solar regulator will simply ‘shed’ this excess voltage, similarly, if the battery bank is full and there is no load on the system then the switched regulator will ‘switch’ off and dissipate the power as heat.

A MPPT charge controller allows all the voltage to be converted to usable power by the means of sophisticated electronics. In turn, all of the voltage and current is converted to usable power, hence greater recharge capabilities to the battery bank and similarly to any electrical load. MPPTs allow any excess voltage, that is any voltage that is higher than the battery bank is converted into voltage.

MPPTs do convert more usable electricity from the solar array into battery recharging and similarly to electrical loads and since that the additional cost may be less than the cost of an additional solar panel, it may be worthwhile in purchasing a MPPT over a switched solar regulator. Below explains more on the technical works of both.

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A conventional switched solar regulator under circumstances where the solar array is not producing the optimal voltages i.e. less than the voltage of the battery bank the system will under perform. This typically occurs in ‘real life’ scenarios where a switched regulator is used. In an Off Grid solar, the solar array and battery bank voltages are ‘matched’ to their typical voltages i.e. a 48 Volt battery bank requires a 48 Volt solar array. This occurs when temperatures of the solar array reduces the voltages to levels that can be less than required for the battery bank to receive an adequate full charge.

A MPPT controller however can convert a higher solar array voltage i.e. more solar solar panels connected in series to the battery bank to increase the voltage in ‘real life’ scenarios. The reduced voltages due to higher temperatures of the solar array become negligible to the charging process because of the number of panels.

The actual panel temperature under normal operating circumstances is considerably higher than the ambient temperature – the higher the panel temperature (anything above 20 degrees) will cause a loss of voltage, known as temperature co-efficient. Typically solar panels can reach 75+ degrees on a summer day, in this instance depending on the quality of the solar panel can result in an approximate loss of 17% of voltage.

For example, in an Off Grid solar system, a 24 Volt solar panel where the Voltage Maximum Power of the panel is 36.6 V at 20 degrees (standard test conditions), at 75 degrees this reduces to the voltage of 30.4 V. The recommended charging voltage for a 24 volt battery is typically between 29 – 31 Volts. In this scenario a switched regulator may not provide sufficient voltage to adequately recharge batteries, especially where the panel temperature is even considerably higher.

A MPPT can accept a lot higher input voltage, hence the possibility of the required recharge loss is eliminated. For example, to avoid the voltage losses due to ‘real life’ scenarios, several panels would be connected together before entering the MPPT i.e. 2 or 3 (73.2 or 109.8 Voltage Maximum Power) and at solar temperatures at 75 degrees 60.7 or 91 V would be obtained, then converted to the required battery voltage.